Malloch Brown was the lead international partner at the US-based Sawyer-Miller Group communications consultancy from 1986 to 1994;[10] he ultimately co-owned the Group with three other partners.[12] The Group was among the first communication consultants to use US-style election campaign methods for foreign governments, companies, and public policy debates.[13] Malloch Brown "worked extensively on privatisation and other economic reform issues with leaders in Eastern Europe and Russia".[12]

In the Philippines, Malloch Brown worked with Corazon Aquino in the campaign against the communist, Malloch Brown wrote Aquino's victory speech which she recited days before voting results were to be released since her campaign assumed that Marcos claim victory as well.[18] He stated that an "outstanding accomplishment during the Cory campaign was to produce an exit poll that indicated that she had won"[19] since other polls released showed that Aquino had been less popular than Marcos.

In 1994, Malloch Brown joined the World Bank as Vice-President for External Affairs, which included responsibility for relations with the United Nations. He used his experience to good effect at the bank, helping to transform its reputation: "under his guidance, the bank blitzed opinion-makers with full-page newspaper advertisements and a television campaign to change perceptions of it as an arrogant institution unwilling to heed outsiders. To his credit, the institution gradually gained a reputation as a 'listening bank', unlike its more aloof sister institution, the International Monetary Fund."[16]

He led the UN's creation of the Millennium Development Goals which were adopted at the UN Millennium Summit in December 2000,[12] later recounting that the draft had gone to the printers without an environmental goal when Malloch Brown passed the head of the UN environment programme in a corridor, leading to the rapid addition of MDG number 7.[20]

In this role Malloch Brown co-ordinated the UN's response to the 2004 Indian Ocean earthquake and tsunami.[8] Internally at UNDP, which was facing increased competition from the World Bank in its areas of responsibility such as capacity building, governance and emergency recovery, he tried to re orient UNDP's activities (sometimes controversially), because of competition with other UN agencies who were also adapting to the demands of a globalising world. Compared with his predecessor, he improved resource mobilisation from donor countries.[citation needed]

In January 2005 he was appointed Chef de Cabinet to UN Secretary-General Kofi Annan, whilst retaining his position as Administrator of UNDP for much of 2005.[12][23]

Malloch Brown publicly defended handling of the Oil-for-Food Programme by the UN in general, and Kofi Annan in particular. While he countered critics that "Not a penny was lost from the organization,"[24] an internal UN audit of the Oil-for-Food programme revealed that there had been overcompensation amounting to $557 million.[25] A separate audit of UN peacekeeping procurement concluded that at least $310 million from a budget of $1.6 billion could not be accounted for.[24]

Malloch Brown, briefing the Security Council, argued that, while the situation uncovered by the audit was "alarming", and that nearly $300 million out of a $1.6 billion budget was involved, it showed more that there was significant waste with only narrow instances of fraud. He noted that the UN Secretariat, based on the reservations expressed by the department being audited, did not entirely accept the auditor's conclusions.[26]

On 6 June 2006, while addressing a conference in New York City, he criticised the United States administration for allowing "too much unchecked UN-bashing and stereotyping". He stated that much of the political dialogue in the US about the UN had been abdicated to its most strident critics, such as conservative talk-show host Rush Limbaugh and the Fox News cable channel and, as a result of this, the true role and value of the UN has become "a mystery in Middle America".[27] These remarks resulted in a backlash from the White House and some US conservative commentators, culminating in a call for an apology by the US envoy to the United Nations John Bolton. Bolton added to reporters, "I spoke to the secretary-general this morning, I said "I've known you since 1989 and I'm telling you this is the worst mistake by a senior U.N. official that I have seen in that entire time."[28]

John Podesta and Richard C. Leone wrote that Bolton's comment "distorted Mr. Malloch Brown's remarks by calling them an attack on 'the American people', and ... by conflating Rush Limbaugh and Fox News with the American people. ...
Mr. Malloch Brown had to break with the niceties of diplomatic tradition to plead for such leadership. ... Mr. Malloch Brown is surely correct: the people of the United States deserve better leadership and diplomacy to represent their interests in the world’s most important international body."[29] Malloch Brown himself rejected the need to apologise, and received the support of Secretary-General Kofi Annan, who said that his deputy's comments "should be read in the right spirit".[30]

In July 2006, during the Israel-Hezbollah crisis in Lebanon, Malloch Brown said America should allow others to "share the lead" in solving the Lebanon crisis, and also advised that Britain adopt a lower profile in solving the crisis, lest the international community see the negotiations as being led by the same team that instigated the invasion of Iraq. These comments again drew criticism from some American officials, including the US State Department, a spokesman from which stated "We are seeing a troubling pattern of a high official of the UN who seems to be making it his business to criticize member states and, frankly, with misplaced and misguided criticisms."[31]

"I don't think the US has anything to object to in the comments. I was really in fact in the interview calling for the US to reach out to France and others to make sure it was demonstrating a broad multilateral coalition and within a single news cycle of my calling for that, it was doing it." He added "I may be prophetic but I wasn't critical".[32]

When Bolton later announced his own resignation in early December, Malloch Brown made his delight clear, telling reporters "No comment – and you can say he said it with a smile".[33]

On 27 June 2007 it was announced that Malloch Brown was joining the Government of incoming Prime Minister Gordon Brown as Minister of State at the Foreign and Commonwealth Office (FCO) with responsibility for Africa, Asia and the United Nations.[16] It was also announced that Malloch Brown would receive a peerage to enable him to sit in the House of Lords; he was also appointed to the Privy Council. Plans for his appointment and peerage had been leaked to The Observer's Pendennis column in November 2006.[34] At the time The Daily Telegraph said that he "While the aid agencies and liberals were still toasting the arrival of 'Saint Mark' to Whitehall, the neo-cons on both sides of the Atlantic were throwing darts at photographs of their devil. [He] divides opinion between those who see him as the great hope for Africa and a principled opponent of the war in Iraq, and those who believe that he is an anti-American egotist who defended Kofi Annan over the oil-for-food scandal."[35]

In November 2007, the conservative British magazine The Spectator drew some attention with its criticism of the Malloch Brown family's occupancy of a government-owned, so-called "grace and favour" apartment in London, previously used by the former Deputy Prime Minister, John Prescott.[38] On 18 November 2007 The Sunday Times fuelled the controversy by reporting that "some see the hand of Miliband behind the savaging of Malloch Brown in The Spectator".[39]

On 7 July 2009, Lord Malloch Brown announced he was stepping down from his position as Minister of State for Africa, Asia and the United Nations at the end of July 2009, citing personal and family reasons.[40][41]

Malloch Brown was appointed chairman of global affairs for FTI Consulting in September 2010.[42] Consultancy appointments to oil companies Vitol and SouthWest Energy Ltd (both approved by the relevant parliamentary committee) were reported in 2010.[43]

Malloch Brown became chairman of the board of directors of SGO Corporation Limited, a holding company whose primary asset is the election technology and voting machine manufacturer Smartmatic, in 2014.[45][46] Citing Malloch Brown's former and present relationship with politicians in the Philippines as well as his role in Smartmatic, the IBON Foundation, a non-profit research organization based in the Philippines, criticized him as being "a foreigner who made a career out of influencing elections".[19]

Malloch Brown was listed 7th in the Leaders & Revolutionaries Section of the Time 100 in 2005.[10][47]

On 15 December 2006, he was named a visiting fellow at the Yale Center for the Study of Globalization and announced plans to focus on writing a book on changing leadership in a globalised world while in residence during the spring semester.[48]

Malloch Brown has been closely associated with billionaire speculator George Soros. Working for Refugees International, he was part of the Soros Advisory Committee on Bosnia in 1993–94, formed by George Soros. He has since kept cordial relations with Soros, and rented an apartment owned by Soros while working in New York on UN assignments.[52] While serving as United Nations Development Fund Administrator, Malloch Brown spoke beside Soros in 2002 suggesting that United Nations and Soros' Open Society Institute, as well as other organizations, work together to fund humanitarian functions.[53]

In 2013 Malloch Brown and FTI Consulting came to a legal settlement with Israeli mining billionaire Beny Steinmetz, who had sued them claiming Malloch Brown had given confidential information to Soros which led to a smear campaign against Steinmetz's mining company. The out-of-court settlement of €90,000 plus costs was without any determination of liability.[57]

Malloch Brown is a member of the Executive Committee of the International Crisis Group,[58] and played a key role in its foundation in 1993-5. In July 2014 he became Co-Chair of the Board of Trustees.[59]